This invention relates to force/torque sensor and more particularly to multi-axis force/torque sensor and the methods of use for directly teaching a task to a mechatronic manipulator.
A mechatronic manipulator is a mechanical device controlled electronically to move or orient an end-effector in space. The term manipulator in this document will refer to mechatronic manipulator having one or more degree-of-freedom (DOF). One category of mechatronic manipulator is a robot manipulator as defined by International Standard ISO 8373 being an “automatically controlled, reprogrammable multipurpose manipulator programmable in three or more axes”.
Manipulators can be used to execute various processes such as arc welding, spot welding, painting, pick and place, assembly, dispensing, polishing or deburring, just to name a few. Each process usually make use of specific type of end-of-arm-tool (EOAT) installed at the robot end-of-arm, also called the robot wrist or face-plate.
Programming a robot typically consists in the three following aspects.                1. Recording various points (positions and/or orientations of the EOAT in space)        2. Adding process parameters: Where the process should start and stop, how the EOAT should be controlled in the sequence, etc.        3. Logic programming, detailing in the program what the robot should do in different circumstances. This logic programming can use information that is internal to the robot and its controller as well as sensor or information from external device(s).        
The most widely used robot programming methods will fall into one of the following categories.                1. Using a teach pendant.                    A teach pendant is a terminal linked to the robot controller. It has a screen as well as several buttons to display and input information. Using the teach pendant buttons, the user jogs the robot to different positions to be recorded, adding process information manually or using pre-defined parameters database. Logic is also added using the robot programming interface on the teach pendant. For very complex tasks, code can be written in the robot specific language or on a computer to later be imported in the teach pendant.                        2. Self-programming from sensor data.                    In this case, the robot will be pre-programmed by experts and the software will be able to make use of sensor data to generate automatically robot path (position sequence), process parameters and logic for a given task.                        3. Off-line programming.                    In this case, the robot and its surroundings are simulated in a virtual environment. Starting from 3D drawings, a software will generate automatically or with little user aid the robot path (position sequence), process parameters and logic. The program is then transferred from the simulation environment to the robot.                        
Current robot programming methods present various challenges to the robot users, mainly because they require a good knowledge of robot kinematics, robot programming language and software. Processes are well known by people in the factories within different industries, the end-users. On the other hand, robots are just a tool for production and how to use them is not necessarily well known by the end-users. Programming robot thus involves a supplemental skill-set that needs to be learned and is also very time consuming, whatever programming method is involved. Using a teach pendant is the simplest but can be very time consuming. Still, the user needs to learn how to program robot on top of knowing the process. Self-programming from sensor data is fast during operations, but implies very complex programming up-front and can only be done initially by advanced robot users. Off-line programming is very efficient at generating complex paths from 3D part drawings and robot models. The differences between the simulated world and the real world need calibration and touch-ups after the generated program has been imported. The user in this case does not only have to learn how to program a robot but also need to learn the off-line programming software, which also implies knowing complex robotic notions.
To cope with the current programming challenges, new robots have been brought to market:                Universal Robots (http://www.universal-robots.com/)        Rethink Robotics Baxter robot(http://www.rethinkrobotics.com/)        Kuka Light Weight Robots (http://www.kuka-robotics.com/en/products/addons/lwr/)        ABB Frida        Industrial robots:                    Assist device U.S. Pat. No. 7,120,508                        
These robots have been specifically designed with the appropriate sensing, mechanical properties and software to allow teaching a task by demonstration. These robots are inherently safe to physically interact with humans during programming. This enables new ways of programming that is closer to teaching in the sense that no code is typed by the user and no position jogging using the teach pendant buttons are used, mainly motion is shown directly to the robot. The manipulator is directly led in space by the user that holds it directly to record the path. This approach simplifies the user learning. How these new robots are built to be inherently safe has the drawback of making them unsuitable for several processes requiring higher stiffness, repeatability, precision, payload, etc.
The current invention aims at reducing programming time and complexity of programming robots and other manipulators of any types, not only the new robots with inherently safe programming human-robot interaction. To this end, a force-torque sensor is installed to an existing robot. Combined with a specific software, this device can interpret the user's intent at moving a robot by holding it. Points and/or paths can then be recorded easily and quickly. Process and logic are also programmed using a simplified user interface on the sensor or elsewhere on the system. To make the solution viable, the sensor need to be cost-effective, robust, present no drift, output a clean signal at a high frequency rate and easy to install in conjunction with the different EOATs and on the different manipulators.
Many multi-axis force sensors have been proposed in the last 30 years. A first attempt to build a multi-axis forces and torques sensor is the “Device for measuring components of force and moment in plural directions” disclosed in U.S. Pat. No. 4,448,083 (publication date: 15 May 1984). In the latter they proposed to measure forces and moments along 3 orthogonal axis using a rigid structure on which is fixed strain gauges to measure local distortion. A variation to this method is the invention disclosed in U.S. Pat. No. 4,763,531 (publication date: 16 Aug. 1988) and named “Force-torque sensor”. This sensor, made for measuring 6 forces and torques component in the cartesian space, is formed with two identically designed, integral spoke wheels. Each of these wheels consist in a rigid cylindrical outer ring and a rigid center hub connected between each other by at least three spokes disposed in a plane. They used a total of 20 strain gauges mounted on theses wheels to provide the 6 components of the force/moment. They claimed that this invention has the advantage to be mounted with exact positioning at very low manufacturing cost. However, while it is an improvement over the first presented approach, the amount of single sensing element required is still very important. More recently, Kang, Dae Im et al. presented the “6-component load cell” in U.S. Pat. No. 5,889,214 (publication date: 30 mars 1999) in which the deformable structure consists of a cross beam having horizontal and vertical parts crossing at right angles. The force measurement is made via many strain gauges attached to the bottom and side surfaces of this cross beam. The precision of this sensor is very high with a maximum interference error 2% in the measuring of force and moment components.
All the above mentioned approaches toward building multiple axis force and moment sensors based their measurements on strain gauges. Although this has the advantage of leading to a very compact and very stiff multi-axis force sensor, this sensing approach also has the drawback of being very sensitive to noise. This is a major inconvenient particularly for use in robotics since most industrial robots generate a lot of electromagnetic noise from the high frequency digital signals used to control their motors. Beside this undesirable noisy characteristic, the strain gauge approach also suffer from the problem of drift of their signal over time. While noise can be filtered, drift is very hard to compensate and in a situation where robot directly react to the level of measured force, drift of the zero force point represented an issue for safety consideration.
In reaction to these drawbacks Hirose has proposed a decade ago to use alternative way to measure forces based on optical sensors as an indication of the displacement of a given compliant structure. This optic based force sensor presented a good immunity to noise, low drift over time and are relatively cheap. However, it is not very accurate for measurement in the nano or low micro scale. Therefore, for a given force measurement range, they must be coupled with a structure of an higher compliance than with strain gauge, a characteristic that can make the resulting sensor less appropriate for some application that require stiffness and precision. An example of a multi axis force and moment sensor based on this approach is the “optical displacement sensor and external force detecting device” disclosed in U.S. Pat. No. 7,220,958 (publication date: 22 may 2007), used three light beams, each projected on a dual photoreceptor. The used of the photodiode technology for force measurement appears to be a very promising avenue, but at this moment still suffer from the short life of photodiode and more, from the constant decrease of its emitting power over its effective life and from the higher structural compliance required in the sensor
One way to circumvent the noise sensitivity of strain gauge based multi-axis force sensor while keeping the sensitivity to very small displacement in the structure is to use the well known relation between the distance and overlap area of two conductive plates and the resulting capacitance measurement.
The proposed invention uses a plurality of sensitive elements for which a conductive plate is positioned on a fixed frame and another one is positioned on a moving frame. The two frames are linked by a compliant element such that the efforts applied on the moving frame will modify the distance between each pair of conductive plates. The positioning of the sensitive elements is completely dissociated from the compliant element. This method has several advantages compared to the existing force/torque sensors (Wacoh [2004, 2006, 2004][U.S. Pat. No. 6,915,709 B2, U.S. Pat. No. 6,915,709, U.S. Pat. No. 6,915,709], Beyeler [US 2009/0007668], Honda [U.S. Pat. No. 7,757,571]). For instance, the positioning of the sensing elements and the design of the compliant element can be optimized separately. As such, the sensing elements can, for example, be placed in a single plane or in an architecture which will maximize the sensitivity, minimize the interrelation between the sensing of two different elements and simplify the calibration methods. Also, because the compliant element is independent from the sensing elements and thus not covered by it, its shape and material are not limited and therefore the relation between the applied forces/torques and the resulting displacements can be further optimized. Finally, because the sensing elements are independent from the compliant element, they can be pre-assembled on a fixture such as a printed circuit board and later be assembled as a whole. This results in the simplification of the sensor fabrication and assembly and the reduction of its cost.